The highly competitive modern game of tennis places ever increasing demands on performance characteristics of rackets. An important aspect relates to racket strength or stiffness versus weight. Modern materials are typically fiber reinforced thermoplastic or thermosetting resins, and lightweight metals or their alloys. In order to save weight with minimum sacrifice in strength, it has long been recognized that it is desirable to provide rackets of hollow, or tubular construction.
The manufacturing problems posed in producing curved hollow structures are exemplified in a paper entitled "Volume Production with Carbon Fibre Reinforced Thermoplastics", R. C. Haines, Plastics and Rubber Processing Application, Vol. 5, No. 1, 1985, which describes the production method for producing a top-of-the-line tennis racket by Dunlop International Sports Company. To produce a hollow frame an injection molding process is used which employs a removable core. A eutectic metal core of a melting point lower than that of thermoplastic resin, is accurately positioned in a mold to ensure uniform wall thickness. Chopped fiber loaded thermoplastic is then injected into the space between the core and the die, and the core is thereafter removed by heating. It should be readily appreciated that such a process is complicated and poses many difficulties.
Other inventions have provided hollow frame configurations which are formed by the assembly and joinder of two or more structural components which define a hollow space in the frame, for example, U.S. Pat. Nos. 4,194,738 and 4,836,543. However, these approaches rely upon the joinder of dissimilar pieces along a curvilinear joint around the racket head. All of these designs present difficulties both from the point of view of mold design and subsequent assembly.